Since the first demonstration of a printed, all polymer transistor in 1994, a great deal of interest has been directed at a potential new class of electronic systems comprising flexible integrated electronic devices on plastic substrates. [Garnier, F., Hajlaoui, R., Yassar, A. and Srivastava, P., Science, Vol. 265, pgs 1684-1686] Recently, substantial research has been directed toward developing new solution processable materials for conductors, dielectrics and semiconductors elements for flexible plastic electronic devices. Progress in the field of flexible electronics, however, is not only driven by the development of new solution processable materials but also by new device component geometries, efficient device and device component processing methods and high resolution patterning techniques applicable to plastic substrates. It is expected that such materials, device configurations and fabrication methods will play an essential role in the rapidly emerging new class of flexible integrated electronic devices, systems and circuits.
Interest in the field of flexible electronics arises out of several important advantages provided by this technology. First, the mechanical ruggedness of plastic substrate materials provides electronic devices less susceptible to damage and/or electronic performance degradation caused by mechanical stress. Second, the inherent flexibility of these substrate materials allows them to be integrated into many shapes providing for a large number of useful device configurations not possible with brittle conventional silicon based electronic devices. Finally, the combination of solution processable component materials and plastic substrates enables fabrication by continuous, high speed, printing techniques capable of generating electronic devices over large substrate areas at low cost.
The design and fabrication of flexible electronic devices exhibiting good electronic performance, however, present a number of significant challenges. First, the well developed methods of making conventional silicon based electronic devices are incompatible with most plastic materials. For example, traditional high quality inorganic semiconductor components, such as single crystalline silicon or germanium semiconductors, are typically processed by growing thin films at temperatures (>1000 degrees Celsius) that significantly exceed the melting or decomposition temperatures of most plastic substrates. In addition, most inorganic semiconductors are not intrinsically soluble in convenient solvents that would allow for solution based processing and delivery. Second, although many amorphous silicon, organic or hybrid organic-inorganic semiconductors are compatible with incorporation into plastic substrates and can be processed at relatively low temperatures, these materials do not have electronic properties capable of providing integrated electronic devices capable of good electronic performance. For example, thin film transistors having semiconductor elements made of these materials exhibit field effect mobilities approximately three orders of magnitude less than complementary single crystalline silicon based devices. As a result of these limitations, flexible electronic devices are presently limited to specific applications not requiring high performance, such as use in switching elements for active matrix flat panel displays with non-emissive pixels and in light emitting diodes.
Progress has recently been made in extending the electronic performance capabilities of integrated electronic devices on plastic substrates to expand their applicability to a wider range of electronics applications. For example, several new thin film transistor (TFT) designs have emerged that are compatible with processing on plastic substrate materials and exhibit significantly higher device performance characteristics than thin film transistors having amorphous silicon, organic or hybrid organic-inorganic semiconductor elements. One class of higher performing flexible electronic devices is based on polycrystalline silicon thin film semiconductor elements fabricated by pulse laser annealing of amorphous silicon thin films. While this class of flexible electronic devices provides enhanced device electronic performance characteristics, use of pulsed laser annealing limits the ease and flexibility of fabrication of such devices, thereby significantly increasing costs. Another promising new class of higher performing flexible electronic devices is devices that employ solution processable nanoscale materials, such as nanowires, nanoribbons, nanoparticles and carbon nanotubes, as active functional components in a number of macroelectronic and microelectronic devices.
Use of discrete single crystalline nanowires or nanoribbons has been evaluated as a possible means of providing printable electronic devices on plastic substrates that exhibit enhanced device performance characteristics. Duan et al. describe thin film transistor designs having a plurality of selectively oriented single crystalline silicon nanowires or CdS nanoribbons as semiconducting channels [Duan, X., Niu, C., Sahl, V., Chen, J., Parce, J., Empedocles, S, and Goldman, J., Nature, Vol. 425, pgs, 274-278]. The authors report a fabrication process allegedly compatible with solution processing on plastic substrates in which single crystalline silicon nanowires or CdS nanoribbons having thicknesses less than or equal to 150 nanometers are dispersed into solution and assembled onto the surface of a substrate using flow-directed alignment methods to produce the semiconducting element of at thin film transistor. An optical micrograph provided by the authors suggests that the disclosed fabrication process prepares a monolayer of nanowires or nanoribbons in a substantially parallel orientation and spaced apart by about 500 nanometers to about 1,000 nanometers. Although the authors report relatively high intrinsic field affect mobilities for individual nanowires or nanoribbons (≈119 cm2 V−1 s−1), the overall device field effect mobility has recently been determined to be “approximately two orders of magnitude smaller” than the intrinsic field affect mobility value reported by Duan et al. [Mitzi, D. B, Kosbar, L. L., Murray, C. E., Copel, M. Afzali, A., Nature, Vol. 428, pgs. 299-303]. This device field effect mobility is several orders of magnitude lower than the device field effect mobilities of conventional single crystalline inorganic thin film transistors, and is likely due to practical challenges in aligning, densely packing and electrically contacting discrete nanowires or nanoribbons using the methods and device configurations disclosed in Duan et al. Use of nanocrystal solutions as precursors to polycrystalline inorganic semiconductor thin films has also been explored as a possible means of providing printable electronic devices on plastic substrates that exhibit higher device performance characteristics. Ridley et al. disclose a solution processing fabrication method wherein a solution cadmium selenide nanocrystals having dimensions of about 2 nanometers is processed at plastic compatible temperatures to provide a semiconductor element for a field effect transistor. [Ridley, B. A., Nivi, B. and Jacobson, J. M., Science, Vo. 286, 746-749 (1999)] The authors report a method wherein low temperature grain growth in a nanocrystal solution of cadmium selenide provides single crystal areas encompassing hundreds of nanocrystals. Although Ridley et al. report improved electronic properties relative to comparable devices having organic semiconductor elements, the device mobilities achieved by these techniques (≈1 cm2V−1 s−1) are several orders of magnitude lower than the device field effect mobilities of conventional single crystalline inorganic thin film transistors. Limits on the field effect mobilities achieved by the device configurations and fabrication methods of Ridley et al. are likely to arise from the electrical contact established between individual nanoparticles. Particularly, the use of organic end groups to stabilize nanocrystal solutions and prevent agglomeration may impede establishing good electrical contact between adjacent nanoparticles that is necessary for providing high device field effect mobilities.
Although Duan et al. and Ridley et al. provide methods for fabricating thin film transistors on plastic substrates, the device configurations described are transistors comprising mechanically rigid device components, such as electrodes, semiconductors and/or dielectrics. Selection of a plastic substrate with good mechanical properties may provide electronic devices capable of performing in flexed or distorted orientations, however, such deformation is expected to generate mechanical strain on the individual rigid transistor device components. This mechanical strain may induce damage to individual components, for example by cracking, and also may degrade or disrupt electrical contact between device components.
Moreover, it is unclear if the plastic substrate-based electronic systems developed by Duan et al., Ridley et al. and others provide mechanical extensibility necessary for many important device applications, including flexible sensor arrays, electronic paper, and wearable electronic devices. While these groups demonstrate electronic devices having the ability to undergo deformation caused by flexing, it is unlikely that these plastic substrate-based systems are capable of stretching appreciably without damage, mechanical failure or significant degradation in device performance. Therefore, it is unlikely that these systems are capable of deformation caused by expansion or compression, or capable of deformation required to conformally cover highly contoured surfaces, such as curved surfaces having a large radius of curvature.
As discussed above, progress in the field of flexible electronics is expected to play a critical role in a number of important emerging and established technologies. The success of these applications of flexible electronics technology depends strongly, however, on the continued development of new materials, device configurations and commercially feasible fabrication pathways for making integrated electronic circuits and devices exhibiting good electronic, mechanical and optical properties in flexed, deformed and bent conformations. Particularly, high performance, mechanically extensible materials and device configurations are needed exhibiting useful electronic and mechanical properties in stretched or contracted conformations.